Many processes use rolled metal stock material as a starting material for manufacturing. Aluminum is exemplary of these roll stock materials and is often subjected to anodization as a processing step. Anodizing is an electrolytic passivation process used to increase the thickness of the natural oxide layer on the surface of metal parts. The process of application is called “anodizing” because the electrical conductive material to be treated forms the anode electrode of an electrical circuit. Anodizing increases corrosion resistance and wear resistance of the material, and provides improved adhesion of coatings. Anodic films can also be used for a number of cosmetic effects, either with thick porous coatings that can absorb dyes or with thin transparent coatings that add interference effects to reflected light.
Anodic films are most commonly applied to protect aluminum alloys. Aluminum alloys are anodized to increase corrosion resistance and to allow dyeing, improved lubrication, or improved adhesion. Anodizing does not increase the strength of the aluminum object and as the tensile strength of the oxide is less than that of the metal, the overall strength is in some instance reduced and the anodized coating is brittle compared to the underlying base material. Also, the anodic layer is non-conductive and this can complicate further processing. Anodization changes the microscopic texture of the surface and changes the crystal structure of the metal near the surface. Thick coatings are normally porous, so a sealing process is often needed to achieve corrosion resistance. Anodized aluminum surfaces, for example, are harder than aluminum but have low to moderate wear resistance that can be improved with increasing thickness or by applying suitable sealing substances. Anodic films are generally much stronger and more adherent than most types of paint and metal plating, but at the expense of comparative brittleness thereby making anodic films more susceptible to cracking from thermal stress.
Although anodizing produces a generally regular and uniform coating, microscopic fissures in the coating can lead to corrosion. Anodization in oxidizing the surface does not in fact planarize an otherwise rough surface. In addition, surface inclusions of other materials that create imperfections in the anodization coating. Further, the anodized coating is susceptible to chemical dissolution in the presence of high and low pH chemistry, which results in stripping the coating and corrosion of the substrate.
Thus, there exists a need for a coating and processes for applying such a coating to rolled stock, such as aluminum, the coating have properties different than those of anodization.